The balancing power that is also described as a reserve power or as a control energy ensures that imbalances (in the case of unforeseeable events) in an electrical power supply grid are redressed. An electrical power supply grid is operated at a nominal grid frequency. This nominal grid frequency is by way of example 50 Hz in the case of the European electrical power supply grid. In the normal grid operation, a maximum grid frequency deviation of by way of example +/−200 mHz is allowed. The balancing power is used to maintain an equilibrium between the electrical power generated in power plants and the electrical power drawn off by the electricity customers including transmission losses. Electrical power supply grids are unable to store energy so that at any point in time the magnitude of supplied electrical power must correspond to the total of the drawn-off electrical power and the power loss occurring as a result of transporting the power. Deviations from this equilibrium in the electrical power supply grid cause a change in the grid frequency that is uniformly synchronous in the entire electrical power supply grid or the electrical power grid. In the case of an oversupply of electrical power, the grid frequency deviates above the nominal grid frequency of the electrical power grid, whereas in the case of an undersupply of power a so-called underfrequency, in other words a grid frequency deviation less than the nominal grid frequency, occurs.
The so-called primary balancing power is used to compensate for any short-term frequency fluctuations. The primary balancing procedure is used to correct imbalances between the physical supply of power and the demand for power and its aim is to restore a stable grid frequency in the electrical power supply grid. Each grid operator within the integrated grid must make available within a specific period of time a certain percentage of their generated power as a primary balancing reserve. The primary balancing procedure is initiated within a few seconds in order to restore the grid frequency of the electrical power network back to the nominal grid frequency of by way of example 50 Hz. More balancing power thus corrects short-term imbalances between the supply and consumption of electricity or electrical power in the electrical power supply grid. The presence of imbalances can be established by way of a frequency deviation with respect to the nominal grid frequency. In the case of an excess supply, in other words in the case of a grid frequency of more than 50 Hz, a negative primary balancing power is provided, whereas in the case of an undersupply, in other words in the case of a grid frequency of less than 50 Hz, a positive primary balancing power is provided. The balancing power is provided by means of technical units that are provided for this purpose and must be provided in a linear manner up to a frequency deviation of +/−200 mHz.
FIG. 1 illustrates the power output into the electrical power grid in dependence upon the grid frequency f for providing a primary balancing power PRL. If the grid frequency f is equal to a nominal grid frequency of 50 Hz, a primary balancing procedure is not performed. A primary balancing power is provided if there is a predetermined frequency deviation interval FAI of +/−200 mHz. If the grid frequency f is less than the nominal grid frequency fnom of 50 Hz, a positive primary balancing power pPRL is provided, wherein electrical power P is fed into the electrical power grid. If, on the other hand, the grid frequency f is greater than the nominal grid frequency fnom, a negative primary balancing power nPRL is provided. As illustrated in FIG. 1, the balancing power RL is regulated in accordance with a linear power characteristic curve in a range between a maximum (curve I) and a minimum power characteristic curve (curve II). The electrical energy that is required for the balancing power can be stored. For this purpose, direct current storage devices or batteries are used in a conventional electrical power grid. The interface between the electrical power supply grid and the energy storage device is formed by means of a converter or an inverter that converts alternating current into direct current and direct current into alternating current. The degree of efficiency of the converter is dependent upon the power. The higher the converted power P, the greater the degree of efficiency of the inverter. In addition, there is a minimum power loss that the inverter or the converter itself consumes in the case of the lowest powers. This minimum power loss is only avoided in the case of a zero power.
FIG. 2 illustrates the degree of efficiency of an energy storage system unit ESS in dependence upon the electrical power P, said energy storage system comprising an energy storage device or a battery and also an inverter. The degree of efficiency of the entire system or of the energy storage system unit is illustrated in the curve I. The curve II illustrates the degree of efficiency of the inverter within the energy storage system unit and the curve III illustrates the degree of efficiency of the energy storage device that is included in the energy storage system unit or the battery that is included in said energy storage system unit.
The operating point AP of an inverter or converter within an energy storage system unit ESS when providing a primary balancing power PRL is in the lower power range (P<1 KW). As is evident in FIG. 2, the degree of efficiency of the inverter is relatively low in this power range, as a consequence of which high electrical losses occur during the operation of the inverter. This behaviour is caused as a result of the fact that small disturbances in the equilibrium occur within the electrical power supply grid considerably more frequently than large disturbances.
Furthermore, in the case of high charge or discharge capacities, the energy storage devices or batteries that are used are heavily loaded which results in the serviceable life of the energy storage devices significantly reducing.
FIG. 3 illustrates the serviceable life of energy storage devices or batteries in dependence upon the number Z of charging or discharging cycles in dependence upon the depth of discharge (DOD) and in dependence upon the electrical power.
The degree of efficiency of the battery and of the energy storage device reduces in addition with the inverter power. This effect is however considerably less than the increase in the degree of efficiency of the inverter with the increasing power P. This effect can therefore be taken into consideration implicitly as a part of the change in the degree of efficiency of the inverter.
Accordingly there is a need to provide a method and a system for providing a balancing power for an electrical power grid, wherein the power loss that occurs is minimized by energy storage system units.